1. Field of the Invention
The present invention relates to a diffraction grating device designed to diffract light of different wavelengths, and to an optical apparatus that transmits and receives light of different wavelengths.
2. Description of Related Art
In optical communication, optical transport members such as optical fibers are used to achieve bi-directional transport of light of different wavelengths. In an optical apparatus used to transmit and receive light to perform such optical communication, i.e., in an optical apparatus that, on one hand, makes light carry signals and then transmits the light to an optical transport member and that, on the other hand, receives light from the optical transport member and then detects the signal carried by the light, it is necessary to share a single optical transport medium to handle both the transmitted and received light but to arrange in different positions a light emitter for transmitting light and a light receiver for receiving light. To achieve this, a splitting/coupling member for splitting and coupling light beams is arranged on an extension line from the optical transport member so that the optical path from the light emitter to the splitting/coupling member and the optical path from the splitting/coupling member to the light receiver are split from each other while the optical paths of those two light beams are coupled together (i.e., made coincident with each other) between the splitting/coupling member and the optical transport member.
To increase communication traffic, an optical transport member is often made to transport light of different wavelengths in the same direction. An optical apparatus of this type is provided with a plurality of light emitters or light receivers, and is further provided with either a plurality of splitting/coupling members or a single splitting/coupling member that has the capability of splitting light of different wavelengths fed from an optical transport member.
A splitting/coupling member is typically realized by the use of a multiple-layer film that reflects or transmits incident light according to wavelength. A multiple-layer film, however, has the disadvantages of requiring a complicated and time-consuming process for the production thereof and being expensive.
The splitting and coupling of light beams needs to be performed not only in an optical apparatus for optical communication but also in an optical recording/reproducing apparatus that uses light to achieve the recording and reading of information to and from a recording medium. Japanese Patent Application Laid-Open No. 2000-163791 proposes the use, as a splitting/coupling member, of a diffraction grating that diffracts incident light at different angles according to wavelength in the optical head of an optical recording/reproducing apparatus that uses light of different wavelengths.
A diffraction grating consists simply of elevations and depressions arranged periodically, and can therefore be produced by resin molding. Accordingly, a diffraction grating device provided with a diffraction grating has the advantage of being suitable for mass production and being inexpensive.
By exploiting the wavelength dependence of the diffraction angle offered by a diffraction grating, it is possible to spatially split a plurality of light beams having different wavelengths. To achieve significant splitting, however, the diffraction grating needs to have the elevations and depressions thereof formed with a small period. Moreover, since the light that is made incident on the diffraction grating to be diffracted thereby is spread within a certain width of wavelengths, even when a parallel light beam is made incident on the diffraction grating, the diffracted light beam inevitably becomes divergent. The divergence of the diffracted light beam is greater the wider the wavelength band of the incident light and the smaller the period of the diffraction grating.
In an apparatus for optical communication, if the diffracted light beam is divergent, part of the light to be transmitted may fail to enter the optical transport member, or part of the light emerging from the optical transport member may fail to enter the light receiver. This results in lower correctness in the signals transmitted and received. To prevent this, optical members for condensing light need to be arranged between the optical transport member and the splitting/coupling member and between the splitting/coupling member and the light receiver. This, however, has the disadvantage of making the apparatus larger.
In an optical recording/reproducing apparatus, if the diffracted light beam is divergent, the light cannot be converged in a very small area on a recording medium, resulting in a lower recording density, or part of the light reflected from the recording medium may fail to enter the light receiver, resulting in lower reading accuracy. To prevent this, the movable objective lens that is arranged between the splitting/coupling member and the recording medium needs to be made larger. This, however, has the disadvantages of making the apparatus larger and lowering the response speed of the objective lens and thus the processing speed of the apparatus.
The diffraction efficiency of a diffraction grating tends to be lower the smaller the period of the elevations and depressions thereof. One way of maintaining high diffraction efficiency while making the period of the elevations and depressions small is to adopt a Littrow arrangement, an arrangement in which the diffracted light beam is closer to the incident light beam than the normal to the diffraction grating at the incidence position. However, in an optical apparatus for optical communication, adopting the Littrow arrangement requires the optical transport member and the light receiver to be arranged spatially close together, making their arrangement difficult.
Moreover, making the period of the elevations and depressions of a diffraction grating smaller results in a greater difference between the diffraction efficiency for the polarization component that is p-polarized with respect to the diffraction grating and the diffraction efficiency for the polarization component that is s-polarized. In optical communication, it is customary to use linearly polarized light to transport signals, and therefore failing to take into consideration the polarization direction of light with respect to a diffraction grating results in lower intensity of the transmitted and received light, leading to lower correctness in the signals transmitted and received.